Electroless nickel plating deposits a nickel alloy onto a substrate which is capable of catalysing the deposition of this alloy from a process solution containing nickel ions and utilising a suitable chemical reducing agent which is capable of reducing nickel ions in solution to metallic nickel. These reducing agents typically include borohydride and hypophosphite ions. Typically, electroless nickel plating is carried out utilizing hypophosphite ions as the reducing agent. As the hypophosphite reduces the nickel at the catalytic surface, some phosphorus is co-deposited with the nickel yielding a nickel/phosphorus alloy containing between about 6 and 12% phosphorus. This alloy has unique properties in terms of corrosion resistance and (after heat treatment) hardness and wear resistance. Common applications of electroless nickel plating include electronics, computers, valves, aircraft parts, and copier and typewriter parts.
In spite of the many advantages of electroless nickel deposits from an engineering point of view, the deposition of electroless nickel generates significant waste. Most of the hypophosphite used to reduce the nickel becomes oxidised to phosphite which remains in the process solution and builds up in concentration until the bath must be replaced. Likewise, the source of nickel in most commercial processes is nickel sulphate, so the process solution also builds up in sulphate ion. During operation of the bath, the pH tends to fall and is corrected either by the addition of ammonia or potassium carbonate solutions. Again, these ions build up in concentration during bath operation. Eventually, the bath reaches saturation (or before this the rate of metal deposition becomes too slow for commercial operation) and has to be replaced. At the point of disposal, the waste solution typically contains nickel ions, sodium ions (from sodium hypophosphite), potassium and/or ammonium ions hypophosphite ions, phosphite ions, sulphate ions and various organic complexants (such as lactic acid or glycolic acid).
During the plating process, the nickel and hypophosphite ions are continuously depleted and must be replenished in order to maintain the chemical balance of the bath. Plating quality and efficiency decrease as the phosphite level increases in the solution, and it becomes necessary to discard the plating bath, typically after the original nickel content has been replaced four times through replenishment. This is known in the art as metal “turnover.”
In order to minimize waste generated from the electroless nickel plating bath, various methods have been developed for precipitating nickel from electroless nickel waste solutions so that the nickel may be recycled and reused in the plating bath. For example, U.S. Pat. No. 4,956,097 to Courduvelis, the subject matter of which is herein incorporated by reference in its entirety, involves decomposing the waste electrolyte at high temperature and separating the precipitated metal. U.S. Statutory Invention Registration No. H1,852 to Alexander et al., the subject matter of which is herein incorporated by reference in it is entirety, suggests precipitating nickel using sodium borohydride at ambient temperature followed by removal of the remaining nickel by precipitation with sodium dimethyldithiocarbamate. U.S. Pat. No. 4,954,265 to Greenberg et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of oxalic acid to precipitate nickel from electroless nickel waste; the resulting liquid is then discharged to a sewerline. U.S. Pat. No. 5,112,392 to Anderson et al., the subject matter of which is herein incorporated by reference in its entirety, describes the removal of nickel from the electroless waste using ion exchange followed by removal of phosphite ions from the nickel-free waste stream using magnesium and calcium oxides. In most cases, the resulting sludge that remains after the nickel is separated is then disposed of in a landfill.
However, it is not believed that any of the above methods addresses the issue of the problems of disposal of the effluent solution after the nickel has been precipitated. Disposal of chemical waste to landfill is becoming more expensive and difficult and may cause damage to the environment. The phosphite ions and ammonium ions present in the electroless nickel waste have a potential use as a fertiliser. Ammonium phosphite is an excellent source of phosphorus which is absorbed through the leaves of plants. However, the presence of sodium and sulphate ions in electroless nickel waste prevents its use on a large scale as a fertiliser intermediate.
The inventors of the present invention have discovered that by utilising electroless nickel solutions based on nickel hypophosphite as a combined source of nickel and reducing agent, it is possible to remove the nickel ions from the waste stream using a suitable cation exchange resin, regenerate the resin using hypophosphorous acid (reforming nickel hypophosphite) and use the resulting solution to manufacture fresh electroless nickel solution. Following nickel removal and recycling, the remaining effluent consists mainly of ammonium phosphite, because no sodium or sulphate ions have been introduced during the operation of the original electroless nickel bath. Thus, the material is suitable for use in fertiliser applications. By a combination of a bath essentially free of sodium ions and sulphate ions with ion exchange recycling technology, the problem of disposal of electroless nickel waste is substantially eliminated.
Although sodium ions are undesirable in the production of fertiliser concentrates, potassium ions are often added as an essential mineral. During the operation of the bath of the invention, it is possible also to incorporate potassium ions, during bath maintenance (by maintaining bath pH with potassium carbonate or hydroxide) and/or during the nickel regeneration step by using the potassium form of the ion exchange resin instead of the acid form.